In terrestrial living systems, water is also the solvent of overwhelming
biological importance. It provides a fluid in which molecules of nutrients
and waste products can be dissolved and transported, helps regulate the
temperature and preserve chemical
equilibrium within living cells, and makes up a major fraction of the
body weight of every organism on Earth.

Other life, other
solvents?

For more than a century, there has been speculation about whether life on
other worlds might use substances other than water to fill the same role.
Many different solvents are known, including hydrogen
sulfide (H2S), the closest chemical analogue of water, and
hydrogen chloride (HCl). But two major problems affect the majority of these
potential rivals. First, they contain elements, such as sulfur
and chlorine, which are much rarer than
the main biological elements of carbon, hydrogen,
oxygen, and nitrogen.
Second, they break up into their constituent atoms more easily. Having said
this, at least two possible candidates for alternative biological solvents
can be identified: ammonia (NH3)
and methanol (CH3OH).

To be effective in sustaining life, a solvent must remain liquid over a
wide temperature range, so that inevitable variations in the conditions
on a planet or moon do not cause the solvent to freeze or boil. This range
should include those temperatures that allow the chemical processes of metabolism
to take place at an optimum rate without disrupting ("denaturing") essential
organic molecules. The solvent should also be able to cushion the organism
against changes in external temperature by having a high heat
capacity and high heat of vaporization.
Finally, it must be able to dissolve a great variety of chemical compounds,
including nutrients and waste materials, so that these can be transported
internally within an organism.

Both ammonia and methanol remain liquid at temperatures well below the freezing
point of water. This might appear to be an advantage on a world that was
generally much colder than the Earth. However, it is not clear how metabolic
processes could take place quickly enough to sustain life at very low temperatures.
Ammonia has much the narrower range of the two and will only remain liquid
at temperatures below minus 33°C. Methanol, by contrast, is liquid
over all of the normal temperature range experienced by terrestrial life,
with the exception of some hyperthermophiles.
Regarding heat capacity, ammonia has the advantage over both water and methanol.
Yet a solvent's ability to act as a thermal buffer, and thereby regulate
the internal temperature of an organism, depends also upon the heat of vaporization.
In this respect, water is far superior to its two rivals. Water's high heat
of vaporization means that only a small amount of evaporation from an organism
is needed to carry away the heat released by cells as a result of their
metabolic processes. Temperature regulation is especially important to the
highest forms of terrestrial life, notably mammals and birds which are homeotherms,
and has been crucial to the rise of intelligent, large-brained animals such
as ourselves (intelligence, nature of).

Some properties of solvents

Solvent

Temperature at which
liquid (°C)

Temperature range

Heat capacity (cal/g-°C)

Heat of vaporization
(cal/g)

Surface tension (water=1)

water

0 to 100

100

1.00

586

1

ammonia

-78 to -33

45

1.23

300

~0.3

methanol

-94 to +65

159

0.60

290

~0.3

Another property of biological significance is surface
tension, a measure of the ability of a liquid to form droplets. Water
has a very high surface tension, a fact which was probably crucial in the
formation of aggregates of organic compounds before cells evolved. It would
have forced some chemicals together while, at the same time, preserving
boundaries between mixtures of different molecules. In living organisms
today it continues to play an important role in concentrating solutions
of substances at the boundaries of different media in an organism; for example,
at a cell membrane.

Of all the features of a solvent, however, the most biologically important
is that which defines it — its ability to dissolve other chemicals.
In this respect, water is superior to both ammonia and methanol by about
a factor of two.